1. Field of the Invention
This invention pertains generally to a nuclear reactor fuel assembly and more particularly to a nuclear fuel assembly that employs a spacer grid that minimizes flow induced vibration.
2. Description of the Related Art
The primary side of nuclear reactor power generating systems which are cooled with water under pressure comprises a closed circuit which is isolated and in heat exchange relationship with a secondary circuit for the production of useful energy. The primary side comprises the reactor vessel enclosing a core internal structure that supports a plurality of fuel assemblies containing fissile material, the primary circuit within heat exchange steam generators, the inner volume of a pressurizer, pumps and pipes for circulating pressurized water; the pipes connecting each of the steam generators and pumps to the reactor vessel independently. Each of the parts of the primary side comprising a steam generator, a pump, and a system of pipes which are connected to the vessel form a loop of the primary side.
For the purpose of illustration, FIG. 1 shows a simplified nuclear reactor primary system, including a generally cylindrical reactor pressure vessel 10 having a closure head 12 enclosing a nuclear core 14. A liquid reactor coolant, such as water, is pumped into the vessel 10 by pump 16 through the core 14 where heat energy is absorbed and is discharged to a heat exchanger 18, typically referred to as a steam generator, in which heat is transferred to a utilization circuit (not shown), such as a steam driven turbine generator. The reactor coolant is then returned to the pump 16, completing the primary loop. Typically, a plurality of the above-described loops are connected to a single reactor vessel 10 by reactor coolant piping 20.
An exemplary reactor design is shown in more detail in FIG. 2. In addition to the core 14 comprised of a plurality of parallel, vertical, co-extending fuel assemblies 22, for purposes of this description, the other vessel internal structures can be divided into the lower internals 24 and the upper internals 26. In conventional designs, the lower internals' function is to support, align and guide core components and instrumentation as well as direct flow within the vessel. The upper internals restrain or provide a secondary restraint for the fuel assemblies 22 (only two of which are shown for simplicity in FIG. 2), and support and guide instrumentation and components, such as control rods 28. In the exemplary reactor shown in FIG. 2, coolant enters the reactor vessel 10 through one or more inlet nozzles 30, flows down through an annulus between the vessel and the core barrel 32, is turned 180° in a lower plenum 34, passes upwardly through a lower support plate 37 and a lower core plate 36 upon which the fuel assemblies are seated and through and about the assemblies. In some designs, the lower support plate 37 and the lower core plate 36 are replaced by a single structure, a lower core support plate having the same elevation as 37. The coolant flow through the core and surrounding area 38 is typically large on the order of 400,000 gallons per minute at a velocity of approximately 20 feet per second. The resulting pressure drop and frictional forces tend to cause the fuel assemblies to rise, which movement is restrained by the upper internals, including a circular upper core plate 40. Coolant exiting the core 14 flows along the underside of the upper core plate 40 and upwardly through a plurality of perforations 42. The coolant then flows upwardly and radially to one or more outlet nozzles 44.
The upper internals 26 can be supported from the vessel or the vessel head and include an upper support assembly 46. Loads are transmitted between the upper support assembly 46 and the upper core plate 40, primarily by a plurality of support columns 48. A support column is aligned above a selected fuel assembly 22 and perforations 42 in the upper core plate 40.
Rectilinearly movable control rods 28, which typically include a drive shaft 50 and a spider assembly 52 of neutron poison rods, are guided through the upper internals 26 and into aligned fuel assemblies 22 by control rod guide tubes 54. The guide tubes are fixedly joined to the upper support assembly 46 and the top of the upper core plate 40. The support column 48 arrangement assists in retarding guide tube deformation under accident conditions which could detrimentally affect control rod insertion capability.
FIG. 3 is an elevational view, represented in vertically shortened form, of a fuel assembly being generally designated by reference character 22. The fuel assembly 22 is the type used in a pressurized water reactor and has a structural skeleton which, at its lower end includes a bottom nozzle 58. The bottom nozzle 58 supports the fuel assembly 22 on a lower core plate 60 in the core region of the nuclear reactor (the lower core plate 60 is represented by reference character 36 in FIG. 2). In addition to the bottom nozzle 58, the structural skeleton of the fuel assembly 22 also includes a top nozzle 62 at its upper end and a number of guide tubes or thimbles 84 which align with the guide tubes 54 in the upper internals. The guide tubes or thimbles 84 extend longitudinally between the bottom and top nozzles 58 and 62 and at opposite ends are rigidly attached thereto.
The fuel assembly 22 further includes a plurality of transverse grids 64 axially spaced along and mounted to the guide thimbles 84 and an organized array of elongated fuel rods 66 transversely spaced and supported by the grids 64. A plan view of a grid 64 without the guide thimbles 84 and fuel rod 66 is shown in FIG. 4. The guide thimbles 84 pass through the cells labeled 96 and the fuel rods occupy the cells 94. As can be seen from FIG. 4, the grids 64 are conventionally formed from an array of orthogonal straps 86 and 88 that are interleaved in an egg-crate pattern with the adjacent interface of four straps defining approximately square support cells through which the fuel rod 66 are supported in the cells 94 in transverse, spaced relationship with each other. In many designs, springs 90 and dimples 92 are stamped into the opposite walls of the straps that form the support cells 94. The springs and dimples extend radially into the support cells and capture the fuel rod 66 therebetween; exerting pressure on the fuel rod cladding to hold the rods in position. The orthogonal array of straps 86 and 88 is welded at each strap end to a bordering strap 98 to complete the grid structure 64. Also, the assembly 22, as shown in FIG. 3, has an instrumentation tube 68 located in the center thereof that extends between and is captured by the bottom and top nozzles 58 and 62. With such an arrangement of parts, fuel assembly 22 forms an integrally unit capable of being conveniently handled without damaging the assembly of parts.
As mentioned above, the fuel rod 66 in the array thereof in the assembly 22 are held in spaced relationship with one another by the grids 64 spaced along the fuel assembly length. Each fuel rod 66 includes a plurality of nuclear fuel pellets 70 and is closed at its opposite ends by upper and lower end plugs 72 and 74. The pellets 70 are maintained in a stack by a plenum spring 76 disposed between the upper end plug 72 and the top of the pellet stack. The fuel pellets 70, composed of fissile material, are responsible for creating the reactive power of the reactor. The cladding which surrounds the pellets functions as a barrier to prevent the fission by-products from entering the coolant and further contaminating the reactor system.
To control the fission process, a number of control rods 78 are reciprocally moveable in the guide thimbles 84 located at predetermined positions in the fuel assembly 22. The guide thimble locations can be specifically seen in FIG. 4 represented by reference character 96, except for the center location which is occupied by the instrumentation tube 68. Specifically, a rod cluster control mechanism 80, positioned above the top nozzle 62, supports a plurality of the control rods 78. The control mechanism has an internally threaded cylindrical hub member 82 with a plurality of radially extending flukes or arms 52 that form the spider previously noted with regard to FIG. 2. Each arm 52 is interconnected to a control rod 78 such that the control rod mechanism 80 is operable to move the control rods vertically in the guide thimbles 84 to thereby control the fission process in the fuel assembly 22, under the motive power of a control rod drive shaft 50 which is coupled to the control rod hub 80, all in a well known manner.
As mentioned above, the fuel assemblies are subject to hydraulic forces that exceed the weight of the fuel rods and thereby exert significant forces on the fuel rods and the assemblies. In addition, there is significant turbulence in the coolant in the core caused by mixing vanes on the upper surfaces of the straps of many grids which promote the transfer of heat from the fuel rod cladding to the coolant. The substantial flow forces and turbulence can result in resonant vibration of the grid straps which results from vortex shedding lock-in vibration when the shedding frequency is close to the natural frequency of the strap. The resonant vibration can cause severe fretting of the fuel rod cladding if the relative motion between the grid strap and the fuel rod is not restrained. Fretting of the fuel rod cladding can lead to a breach and expose the coolant to the radioactive byproduct within the fuel rods. Another potential problem with resonant grid strap vibration is that fatigue could occur in the grid straps causing grid strap cracking (or other damage to the straps).
Thus, an improved means of supporting the fuel rods within a fuel assembly grid is desired that will better resist resonant vibration of the grid straps.